1. Field of the Invention
The present invention relates to a hollow rack shaft, particularly relates to a hollow rack shaft used for a steering system of an automobile.
2. Description of the Related Arts
A hollow rack shaft used for a steering system of an automobile and its manufacturing method are disclosed in Japanese Laid-Open Patents No. Hei 11-180318 and No. Hei 11-278287 and others. In any manufacturing method, rack teeth are formed at the bottom of a U-shaped plate bent in a gutter-like shape, and afterward, the plate is formed cylindrically by butting two open legs of the U shape. The butt portion of the cylinder is normally welded to acquire higher strength except a case that there is no problem on the strength and this cylinder is used as a hollow rack shaft.
FIGS. 1a and 1b are a top view and a sectional view showing a workpiece for manufacturing the above shaft. FIGS. 2a to 6c show the forming process from the material to the hollow rack shaft in this manufacturing method. These drawings accompanied by a, b and c are respectively equivalent to a front view of the rack shaft (the workpiece), a Dxe2x80x94D sectional view of the part in which rack teeth are formed, and an Exe2x80x94E sectional view of the part in which no rack tooth is formed.
The workpiece of a substantially rectangular plate shown in FIGS. 1a and 1b, is bent into a gutter-like shape having a U-shaped part with a flat bottom in an area A1, and having a U-shaped part with a semicircular bottom in an area A2 on both sides of A1 as shown in FIGS. 2a to 2c. After a row of rack teeth is formed at the flat bottom of the gutter-shaped workpiece as shown in FIGS. 3a to 3c, the ends of two open legs of the gutter-shaped workpiece are bent inside so that they are joined as shown in FIGS. 4a to 4c. The section of the part in the area A2 becomes circular by this bending In most of cases, after the ends of these legs are mutually welded at butt portion as shown in FIGS. 5a to 5c, the workpiece becomes a hollow rack shaft shown in FIGS. 6a to 6c after finishing process.
Generally, welding has the cooling contraction problem which caused by the solidification and the contraction of melted and expanded material at cooling. Internal stress is caused by the contraction through cooling of welded part, and distortion is also caused in the whole workpiece.
FIG. 7 exaggeratingly shows the hollow rack shaft deformed in whole by the contraction through cooling of the welded part 5. As the butt-welded part 5 is located on the reverse side to the rack teeth, the warp that the contracted welded part (a weld bead) 5 is on the inside and the row of rack teeth is on the outside is caused.
At this time, as the row of rack teeth is also influenced by the deformation, the rack teeth are out of the standard of teeth profile and cannot be substantially used for a rack shaft as they are. That is, as the rack shaft also warps in the area A1 having the rack teeth and the precision of the teeth profile is deteriorated by this effect, the rack shaft cannot be normally engaged with a pinion.
Such deviation in the precision of the teeth profile of the rack can be rectified by finishing through grinding in another process. However, as the merit that plastic working has excellent productivity is lost, it is not a good policy to insert grinding process in order to rectify the teeth profile in the manufacturing process on the hollow rack shaft. Further, as welding requires the process to remove the weld bead, the number of processes and the cost for manufacturing the hollow rack shaft are increased.
Further, as the rack teeth are designed as thin as possible in consideration of the lightening, it cannot be allowed in view of the strength that the teeth are further ground by grinding.
The hollow rack shaft has further another problem. As a pinion coupled to a steering shaft is engaged with the rack teeth and steering force is transmitted from the pinion, large bending stress acts upon the dedendum of the rack teeth and high contact pressure acts upon the contact part of the surface of the rack teeth. To endure the large bending stress and the high contact pressure, the rack teeth are generally hardened up to approximately the hardness Hrc 60 by carburizing quenching or induction hardening. In the meantime, such high contact pressure and large bending stress do not act upon the part in which no rack teeth are formed.
Nevertheless, a workpiece of one thick plate made of high-priced material is used to suit to the largest load as a whole, and further, the rack shaft which is made in this method has the same diameter of the area A1 that the row of rack teeth is formed as that of the areas A2 on both sides of the area A1. As described above, in prior art, superfluous quality is given to the area A2 which receives only small load in view of the material and the dimension, and therefore, the manufactured hollow rack shaft does not achieve enough lightening and it is high-priced.
The invention provides a hollow rack shaft and its manufacturing method wherein a process for a butt weld is not required, therefore, the process of weld beads and the rectification of the welding warp are not required and in addition, the strength is the same as what is welded.
Further, the invention has the object of acquiring a low-priced and light hollow rack shaft, particularly a hollow rack shaft for a steering system and manufacturing it easily. Furthermore, the invention has an object of using an enough thick and high quality plate in order to apply enough strength to rack teeth, using a thin plate made of low-priced material for part having no rack teeth, acquiring a light and low-cost hollow rack shaft and hereby further, acquiring the useful hollow rack shaft having suitable quality a-d the balance of the strength as a whole.
A workpiece for the hollow rack shaft disclosed in this specification is a substantially rectangular plate. The opposite two sides of this workpiece have complementary profiles each of which is composed of the continuation of a convex portion and a concave portion. When the workpiece is formed cylindrically, the profiles are mutually engaged. As the size of the widest part of the convex portion is wider than that of the narrowest part of the concave portion, the above-mentioned two sides are prevented from being detached after engagement. Further, the opposite convex portion and concave portion are mutually caulked, and hereby, respective gap is filled up.
A workpiece for another hollow rack shaft disclosed in this specification is one plate acquired by welding a first plate for a rack teeth area for a row of rack teeth to be formed and a second plate for the other area. For the first plate, a plate thicker than the second plate and/or a plate made of more satisfactory material in hardenability are/is used.
Further another hollow rack shaft disclosed in this specification is provided with a first area and a second area on both sides of it. In the first area, a row of rack teeth and a semi-cylindrical part on the reverse side are formed, the second area is fully cylindrical and on at least one side of the second area, apart different from the semi-cylindrical part in the first area and having a particularly small diameter is formed.
Other objects and advantages besides those discussed above shall be apparent to those skilled in the art from the description of a preferred embodiment of the invention which follows. In the description, reference is made to accompanying drawings, which form a part thereof, and which illustrate an example of the invention. Such example, however, is not exhaustive of various embodiments of the invention, and therefore reference is made to the claims which follow the description for determining the scope of the invention.